1. Field of the Invention
The invention relates to a microporous pressed molded thermal insulator body, in particular for high-temperature applications, substantially comprising a base body made of a highly dispersed silica aerogel and/or metal oxides with fiber reinforcement and opacifier and a cover out of shrinkable plastic foils.
2. Brief Description of the Background of the Invention Including Prior Art
A method for the production of such thermal insulator bodies is described in the German printed patent document DE-C2 4,106,727. According to the description, a partial or a complete cover made of a shrinkable plastic foil is shrunk onto the pressed base body, where the base body was pressed beforehand without a cover. The base body can also be covered with a metal foil. For balancing the high thermal conductivity of the metal cover, the base body must, naturally, be made thicker, which impairs the flexibility of the thermal insulator bodies.
A thermal insulator body, made of a mixture of pyrogenic silica and opacifier, is known from the German printed patent document DE 2,729,609-A1. The mixture is in this case continuously entered and filled into a cover or envelope, made of a flat material such as a tube, and the filled tube is subdivided without separating crosswise to the longitudinal direction of the tube and is pressed under heat application onto the cover to a strand or series of plates. The tube is to be formed according to one embodiment of the method out of two individual bands of the flat material, running together in the region of the filling of the flat material by joining at the side edges.
The German printed patent document DE 2,712,625 A1 describes a method for the production of insulating plates out of a mixture of finely dispersed silica and a clouding agent. The finely dispersed silica is in this case filled into a sack or bag, and the cover is brought to a predefined temperature during the pressing process such that the cover afterwards surrounds the core material under tension. Shrink foils are employed for obtaining thermal insulating plates against cold, and glass fiber is employed for covers for the thermal insulating plates against heat. Glass fibers or, respectively, glass fabrics have a low bending strength and a low wear stability and become brittle at 500.degree. C. The latter limits the application of the thermal insulating plates made according to this method to low temperature regions. In addition, the powdery thermal insulating material can be easily anchored in its rough and porous surface such that bending applications (be it in case of a one-sided thermal application or during bending) result in breakage. This can even occur already during the shrinking process.
This holds also for the thermal insulator bodies, enveloped with a fabric sack or bag made of quartz fibers, as described in the German printed patent document DE 2,928,695-A1. According to this printed document, a sliding agent in the shape of dispersions, such as emulsions or suspensions, pastes or powders, is furnished for avoiding these disadvantages. These pastes or powders are applied to the inner side of the cover which is cumbersome and time-consuming and which makes the product substantially more expensive. In addition, the applied layer can easily form lumps during the filling of the thermal insulating material and thus cause rejects. In particular, however, the separating material can displace the pores of the adjoining material layers such that a lesser thermal insulation has to be accepted. This could in fact be avoided and prevented by a correspondingly larger thickness of the plates, however, this again would not allow the occurrence of larger bending angles.
Since finely dispersed powders, such as a mixture of pyrogenic silica and clouding agent, are not capable to flow, i.e. are not free-flowing, according to the teachings of the German printed patent document DE 2,928,695-A1, a uniform pressing up into the edge zones does not occur during the pressing into bag-like covers such that a uniform densification is not assured, and in particular the edges can crack and crumble off in case of thermal insulator bodies produced according to this method. Cracked edges usually result in a breaking apart of the thermal insulator body. In addition, based on non-uniform densification and the associated thickening, the cover does not cling at all. Places such that hollow spaces, leading to a crack formation, are generated.